If the Slavs remain on the north side of the Danube, they would be in a position to form a nation within the Pannonian Plains. Although one question: would the Bulgar conquest be butterflied away?
I think any Slavic polity would probably form around the Carpathians and Wallachia. The evidence is
extremely hazy, but I've read that the latest scholarship suggests eastern Romania, Moldova and western Ukraine are probably the most likely areas for a Slavic homeland. More likely you'd see a number of smaller Slavic kingdoms developing over the course of the seventh and eighth centuries.
Most likely. If Byzantium was at its Pre-Sassanid war strength, it would absolutely destroy them.
True, but the Huns and Avars were able to do fairly well against the later Roman state. Don't forget!
Anyway does anyone have any idea on what would happen if the Byzantines just didn't attempt expand at all during the Justinian period?
I think an earlier form of "Great Schism" is quite likely without the Empire being directly involved in the politics of the city of Rome: Justinian will be much more able to make more theological compromises with the anti-Chalcedonians than he was IOTL when he had to worry about bringing along the Italian and African bishops too. I'd say an earlier form of Monothelitism is quite likely.
In Italy, the Ostrogoths are more likely than not to convert to Chalcedonian Orthodoxy at some point, probably in the second half of the sixth century. The Vandals might stick to Arianism for a while longer, but I'd be surprised if they were still Arian by 650.
Islam is obviously butterflied by this, but the bubonic plague is not, so the Empire isn't going to be noticeably stronger by Justinian's death than it was IOTL: I'm of the opinion that blaming the Western wars for a decline in imperial power is a bit of a chimera given they were basically done on the cheap and only in Narses' final (and totally successful) campaign of 552 was serious money and manpower put into them. Arguably the northern Balkans suffered, but this was a region that had been already quite badly ravaged by the Goths in the 370s and 380s, Huns in the 440s, and then Ostrogoths in the 480s. Roman civic life in the region had retreated to fortified strongholds long before Justinian was even born.